Cable bolts are usually made from cable comprising a plurality of steel filaments wound together around a central wire to form a tendon. Resin and/or cement grout is used to fix the cable bolt to a borehole. To increase the effective bond strength between the cable bolt and resin or grout the bolts are often provided with spaced protuberances along the length of the cable. These protuberances are often known as bulbs or cages. The protuberances assist in preventing cable bolts from being pulled through the resin or grout, thus providing improved anchorage and load transfer between the cable, resin/grout and the surrounding strata.
It is known that tensioning of the cable prior to cement grouting can cause the protuberance to collapse thus reducing the cable's effectiveness. In Australian patent 2004260817 there is a proposal to insert ball bearings into the cavities defined by the protuberances to reduce the likelihood of the protuberances collapsing when the cable is tensioned. This proposal has proved expensive to manufacture and unreliable due to the ball bearings being pushed out of the protuberances. There is also a need to displace the central wire to locate each ball bearing. In some cable bolts the central wire is replaced by a hollow tube which extends along the centre of the cable. Other disadvantages relate to the difficulty in automating the placement of the ball bearings and the ball bearing creates a stress concentration on the strands of the cable creating loads that lead to failure loads up to 25% less than the original strands ultimate tensile strength.
In our earlier Australian patent application 2008200918 we disclose a cable bolt having a hollow strand which facilitates the passage of grout along the cable. It is important that the hollow strand does not get crushed by radial loads in non collapsible protrusions.
It is these issues that have brought about the present invention.